Today, nonaqueous electrolytic cells represented by lithium secondary batteries have been employed widely for consumers use such as small-sized portable appliances as nonaqueous electrolytic cell with high energy density. Recently, for new uses, it has been expected to manufacture middle-sized and large-sized nonaqueous electrolytic cells and apply them as electric power storage facilities and motive energy for vehicle such as HEV.
In manufacturing middle-sized and large-sized nonaqueous electrolytic cells, high reliability is required. In general, for a nonaqueous electrolytic cell, a positive electrode using a transition metal oxide as a positive active material, a negative electrode using a carbon material as a negative active material, and a nonaqueous electrolyte containing an electrolytic salt such as LiPF6 dissolved in a nonaqueous solvent such as carbonate are used, and since insertion/extraction reaction of lithium ion into/from the carbon material of the negative electrode is mainly caused at a less potential than the reduction decomposition potential of the nonaqueous electrolyte, although the energy density is heightened, there is contrarily a weak point in terms of the reliability such as the life or high temperature property.
In hope of improvement of the reliability, proposed is a nonaqueous electrolytic cell using lithium titanate as a negative active material into/from which insertion/extraction reaction of lithium ion is cause at a nobler potential (around 1.5 V) as compared with a carbon material. However, when lithium titanate is used as a negative active material, in the initial charge-discharge process during the manufacturing step, gas generation is caused mainly due to reaction of lithium titanate and a nonaqueous solvent. If the gas generation reaction is caused, the power characteristic and life characteristic of the cell are worsened because of the characteristic change of the electrode surface due to the decomposition reaction of the liquid electrolyte and change of the physical property and the amount of the liquid electrolyte. Further, it becomes a cause of the cell swelling.
It is unfavorable for a nonaqueous electrolytic cell to be penetrated with water in the inside, and therefore, in the manufacturing step, a huge installation investment is required to keep a container being left in opened state for a long time without being closed after a nonaqueous liquid electrolyte filling into the container and it is not practical, and thus tight sealing is strongly required immediately after the nonaqueous liquid electrolyte filling.
Nonaqueous electrolytic cells using lithium titanate as a negative active material is currently commercialized as products mainly for back up use (e.g., reference to Non-patent Documents 1 and 2, coin-type lithium secondary cell (Sony)) and they are coin type cells having a capacity at most about 20 mAh and the maximum current of about 0.5 ItA. With respect to coin type cells with a small capacity, because, for example, containers are strong, the gas generation during the manufacturing step does not become a big issue. However, in a case of manufacturing a middle-sized, large-sized, and large capacity nonaqueous electrolytic cells using lithium titanate as a negative active material, the gas generation becomes a problem which is nonnegligible. The reasons for that are because the installation investment scale becomes high in the case a section where the open state is kept after the nonaqueous liquid electrolyte filling exists in the manufacturing line; because the container tends to be susceptive to swelling along with the enlargement of the surface area of the container; because the container also tends to be susceptive to swelling due to increase of the equilibrium point of the internal pressure in a case a resin sealing agent which can permeate some of gases is used; and so forth. Herein, middle sized, large-sized, and large capacity cells means cells with 10 mAh or higher, preferably 100 mAh or higher, and more preferably 200 mAh or higher.    Non-patent Document 1: Journal of Power Sources 146 (2005) 636-639    Non-patent Document 2: Shingakugiho EE2005-50 CPM2005-174
As a method for moderating the effect of swelling or the like due to gas generation in middle-sized and large-sized cells, a method of keeping a large dead space in cells can be exemplified; however such a cell design is contradictory to the designing concept of high energy density cells. From this viewpoint, it is proper to keep the dead space calculated by subtracting the volume of the solid matter and liquids such as a power generating element, an electrode element, and a liquid electrolyte from the internal content of a container to 35% by volume or less.
With respect to middle-sized, large-sized, and high capacity nonaqueous electrolytic cells using lithium titanate as a negative active material, it has been required to develop techniques of not only suppressing gas generation in the initial charge-discharge step carried out in the manufacturing step but also suppressing gas generation of cells after completion, particularly gas generation at the time of high temperature storage and thereby suppressing swelling in a nonaqueous electrolytic cell.
Since gas is generated in the electrode surface, it is apparent that the problem can be solved if an ideal coat is formed on the electrode surface; however, any conventional techniques cannot accomplish formation of the ideal coat. For example, if a firm coat of polyethylene is formed on the electrode surface, hydrogen gas generation can almost completely be suppressed; however the electrode reaction is significantly inhibited and the cell performance is thus extremely worsened. As described, the ideal coat should not be so vainly dense or vainly thick as to inhibit electrode reaction and is required not to increase the thickness or disintegrate or generate hydrogen gas or other gases.
Various means for suppressing gas generation in nonaqueous electrolytic cells using lithium titanate as a negative active material have been proposed. For example, Patent Document 1 proposes optimization of a carbonaceous material being an electric conductive agent. Further, Patent Document 2 proposes use of a nonstoichiometric titanium oxide as an electric conductive agent. Furthermore, Patent Document 1 describes “examples of the organic solvent may include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and vinylene carbonate (VC); linear carbonates such as dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), and diethyl carbonate (DEC); cyclic ethers such as tetrahydrofuran (THF) and 2-methyltetrahydrofuran (2MeTHF); linear ethers such as dimethoxyethane (DME); γ-butyrolactone (BL), acetonitrile (AN), and sulfolane (SL)” (paragraph 0032), and Patent Document 2 also describes the same (paragraph 0062); however specifically, only “a solvent mixture of ethylene carbonate (EC) and γ-butyrolactone (BL)” (volume ratio 25:75) (paragraph 0053 in Patent Document 1 and 0074 in Patent Document 2) is disclosed and use of vinylene carbonate is neither specifically described nor formation of a coat on negative electrode surface and therefore, it is not easy for a person skilled in the art to form a coat on the negative electrode surface by using a nonaqueous electrolyte containing vinylene carbonate on the basis of Patent Documents 1 and 2.
Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2005-100770
Patent Document 2: JP-A No. 2005-332684
Patent Document 3 describes “lithium titanate and diethylene glycol dimethyl ether are reacted by using a negative electrode containing lithium titanate as an active material and a nonaqueous liquid electrolyte containing diethylene glycol dimethyl ether to form an ion conductive coat on the surface of lithium titanate of the negative electrode. It is considered that the storage property of a lithium secondary cell of this invention is improved by suppressing reaction of lithium titanate, which is an active material, with the nonaqueous liquid electrolyte” (paragraph 0006) and thus shows a technical idea to form the coat and improve the storage property by using solvent decomposed at a relatively nobler potential at which lithium titanate acts at the same time. Further, also disclosed is the lithium secondary cell in which the solvent of the liquid electrolyte comprises a solvent mixture of propylene carbonate and diethylene glycol dimethyl ether (claim 3). However, as shown in Examples of present description, even if the solvent mixture of propylene carbonate and diethylene glycol dimethyl ether is used, the gas generation and cell swelling cannot be suppressed sufficiently.
Patent Document 3: JP-A No. 2004-95325
Patent Document 4 proposes improvement of the cycle performance and storage characteristics by using a separator containing mainly polyphenylene sulfide or polyether ether ketone. Further, Patent Document 5 proposes improvement of the cycle life by using fluorinated lithium-containing titanium oxide. Moreover, Patent Document 4 describes “since polyphenylene sulfide (PPS) or polyether ether ketone (PEEK), main materials of the separator, are excellent in the chemical stability, reaction with lithium titanate or titanium oxide having high reduction ability is scarcely caused. In addition, cyclic carbonates and linear carbonates which are nonaqueous solvents form chemically stable coat on the negative electrode surface by reaction with lithium titanate or the like. Accordingly, deterioration of cell performance due to reaction of the separator with lithium titanate or titanium oxide during storing cell can be prevented” (paragraph 0009) and “examples of the cyclic carbonates include ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate and examples of the linear carbonates include dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate” (paragraph 0011); however use of vinylene carbonate as a cyclic carbonate is not specifically described (reference to Table 3) and further only the capacity retention ratios after storage are shown but suppression of gas generation at the time of storage is not referred to and therefore on the basis of the descriptions of Patent Documents 4 and 5, it cannot be said that a person skilled in the art could have easily achieved the suppression of gas generation at the time of storage by previously forming the coat on the negative electrode surface by using a nonaqueous liquid electrolyte containing vinylene carbonate.
Patent Document 4: JP-A No. 2004-87229
Patent Document 5: JP-A No. 2005-302601
Patent Document 6 describes “there is no problem to use a nonaqueous electrolyte secondary cell comprising lithium titanate as a negative active material as a main power supply for portable appliances; however there occurs a problem of cell performance deterioration in the case of using this nonaqueous electrolyte secondary cell as a power supply for memory back up at operating voltage of around 3.0 V. The reason for that is because when such a nonaqueous electrolyte secondary cell is used as a main power supply of portable appliances, the negative electrode is charged up to around 0.1 V on the basis of lithium metal at the time of charging, and therefore, a coat excellent in ion conductivity is formed on the surface of the negative electrode, reaction of the negative electrode and the nonaqueous liquid electrolyte is suppressed due to the coat, and thus decomposition of the nonaqueous liquid electrolyte and breakage of the negative electrode structure can be prevented. On the other hand, when this nonaqueous electrolyte secondary cell is used as a power supply for memory back up at operating voltage of around 3.0 V, charging with very small current about 1 to 5 μA is carried out while the constant voltage state around 3.0 V is kept for a long time and charging of the negative electrode is carried out up to only about 0.8 V on the basis of lithium metal and therefore, the coat is not formed on the surface of the negative electrode and thus the negative electrode and the nonaqueous liquid electrolyte are reacted to decompose the nonaqueous liquid electrolyte or break the negative electrode structure” (reference to paragraphs 0006 and 0007) and thus it is described that when the nonaqueous electrolyte secondary cell using lithium titanate as the negative active material is charged up to around 0.1 Von the basis of lithium metal, the reaction of the negative electrode and the nonaqueous liquid electrolyte can be suppressed due to the coat formed on the surface of the negative electrode; however, it is not described about use of the cell having the coat on the negative electrode surface charged up to around 0.1 V in a range of negative electrode potential nobler than the lithium potential by 0.8 V and contrarily, according to description of Patent Document 6, the cell to be used in a range of negative electrode potential nobler than the lithium potential by 0.8 V is assumed to have no coat on the negative electrode surface and accordingly, use of the cell having the coat on the negative electrode surface in a range of negative electrode potential nobler than the lithium potential by 0.8V is rather inhibited and therefore a person skilled in the art could have easily achieved the use. Further, as below-described in Examples of the present description, in the case of using the cell having the coat on the negative electrode surface in a range of negative electrode potential around 0.2 V relatively to the lithium potential, suppression of gas generation and suppression of cell swelling are insufficient and therefore, it could not be expected that gas generation and cell swelling are remarkably suppressed by using the cell having the coat on the negative electrode surface in a range of negative electrode potential nobler than the lithium potential by 0.8 V.
Patent Document 6: JP-A No. 2005-317509
Further, there is a description in same document as “with respect to the nonaqueous electrolyte secondary cell of the invention, as the nonaqueous solvent to be used for the nonaqueous electrolyte, generally employed and known nonaqueous solvents can be used and particularly, solvent mixtures obtained by mixing cyclic carbonates and linear carbonates are preferably used. Herein, examples of the cyclic carbonates include ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate. Examples of the linear carbonates include dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate. Further, as the nonaqueous solvent, γ-butyrolactone and solvent mixtures obtained by mixing γ-butyrolactone and cyclic carbonates can be also used. Since the cyclic carbonates are generally easy to be decomposed at a high potential, the ratio of the cyclic carbonates in the nonaqueous solvent is preferably in a range of 10 to 50% by volume and more preferably in a range of 10 to 30% by volume. In the case of using ethylene carbonate as a cyclic carbonate, the storage property is excellent” (paragraph 25); however as described “in the case of using a lithium transition metal composite oxide defined by a general formula LiMnxNiyCozO2 (x+y+z=1; 0≦x≦0.5; 0≦y≦1; 0≦z≦1) is used as the positive active material in the positive electrode, if the weight ratio of the negative active material to the positive active material is set to 0.57 or higher and 0.95 or lower, the voltage at the time of finishing charging in the negative electrode is around 0.8 Von the basis of lithium metal when the charging is carried out while keeping constant voltage state at around 3.0 V, and it is thus suppressed that the nonaqueous liquid electrolyte is decomposed because of reaction with the negative electrode or the negative electrode structure is broken . . . ” (paragraph 0022) and accordingly since “the voltage at the time of finishing charging in the negative electrode is around 0.8 V on the basis of lithium metal”, no coat is formed on the surface of the negative electrode in accordance with the description in paragraph 0007 of the same Document and it cannot be said that a person skilled in the art could have easily achieved the suppression of gas generation by forming the coat on the negative electrode surface by using a nonaqueous electrolyte containing a cyclic carbonate such as “ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate” on the basis of the description of the same Document.
Patent Document 7 discloses that the conductivity of an electrode containing Li4/3Ti5/3O4 is improved by adding a compound having S═O bond such as propane sultone or ethylene sulfide in order to improve the high load discharge characteristics of a nonaqueous electrolyte secondary cell using a lithium titanium compound as a negative active material. However, as shown in Examples of the present description, even if propane sultone (1,3-propane sultone) is added alone to the nonaqueous electrolyte, gas generation or cell swelling cannot be suppressed sufficiently.
Patent Document 7: JP-A No. 2003-163029
Furthermore, aiming suppression of gas generation due to decomposition of propylene carbonate occurring in the case of using propylene carbonate for a nonaqueous electrolyte in a nonaqueous electrolytic cell having a negative electrode of a carbon material, many proposals of adding various kinds of additives have been made (e.g. reference to Patent Document 8).
Patent Document 8: JP-A No. 2005-11768
Patent Document 9 also discloses an invention, “a lithium secondary cell comprising a positive electrode using a composite metal compound of lithium and at least one metal selected from a group consisting of chromium, vanadium, manganese, iron, cobalt and nickel as positive electrode material, a negative electrode using a carbon material as a negative electrode material, and a liquid electrolyte obtained by dissolving an electrolyte in a nonaqueous solvent, wherein the nonaqueous solvent contains not less than 10 wt. % and not more than 60 wt. % of propylene carbonate, not less than 30 wt. % and not more than 80 wt. % of at least one or more linear carbonates selected from methyl ethyl carbonate, methyl propyl carbonate, and methyl butyl carbonate, and not less than 0.01 wt. % and not more than 5 wt. % of vinylene carbonate (VC)” (claim 1); however this invention aims to solve a problem “with respect to a PC type liquid electrolyte in a lithium secondary cell using graphite with high crystallinity as a negative electrode material, it is disadvantageous that PC in the liquid electrolyte is decomposed by graphite during charging and thus fails to obtain good cycle performance” (paragraph 0003) and also there is a description “the invention is accomplished based on the unexpected fact that a liquid electrolyte obtained by selecting PC (freezing point −55° C.) having rather much lower freezing point than those of EC and VC as a high dielectric constant solvent and dissolving an electrolyte in a nonaqueous solvent of a linear carbonate with further low viscosity and VC does not cause PC decomposition even in the case of graphite negative electrode and shows remarkably excellent cell performance even at a low temperature” (paragraph 7) and therefore, the nonaqueous solvent is employed to solve the particular problem for a nonaqueous electrolytic cell using a carbon material for the negative electrode (the aim for suppressing cell swelling is not described) and it is not suggested to apply the nonaqueous solvent to a nonaqueous electrolytic cell using a material other than a carbon material for a negative electrode.
Patent Document 9: Japanese patent No. 3632389
Further, Patent Document 10 discloses an invention “a nonaqueous electrolytic cell comprising a rolled flat type power generating element having a positive electrode, a separator, and a negative electrode containing a carbon material as well as a liquid electrolyte in a cell case made of a metal-laminated resin sheet, wherein a solvent of the liquid electrolyte is a solvent mixture of vinylene carbonate, propylene carbonate, and a linear carbonic acid ester and satisfies the expression: 10≦(A+B)≦50 (wherein A≠0 and B≠0) and 3≦A≦20, wherein A denotes the composition ratio by vol. % of the vinylene carbonate to the total solvent and B denotes the composition ratio by vol. % of the propylene carbonate to the total solvent” (claim 1) and this invention aims to solve a problem “as compared with those made of conventionally used metals and having high rigidity, the laminate case is weak to the outer force and tends to be deformed. Therefore, particularly in the case of standstill at a high temperature, excess gas is generated in the cell due to vaporization of the liquid electrolyte or electrochemical decomposition or thermal decomposition of the liquid electrolyte by oxidation or reduction of the positive electrode/negative electrode active material surface, and accordingly the cell using the laminate case is expanded and deformed due to increase of cell internal pressure” (paragraph 0005), and there are descriptions “on the other hand, since lithium type cell can obtain high voltage, it is desirable to select a liquid electrolyte excellent in the withstand voltage characteristics and propylene carbonate can be exemplified as its candidate, however in the case a carbon material is used for the negative electrode, propylene carbonate is decomposed. Accordingly, for the liquid electrolyte of the nonaqueous electrolytic cell using the carbon material for the negative electrode, use of propylene carbonate is improper although it is advantageous as the liquid electrolyte” (paragraph 0006), and “in above-mentioned Examples, although graphite is used as a substance absorbing and desorbing lithium, which is a negative electrode material, the negative electrode material is not limited thereto, but any carbon materials can be used as the negative electrode material as long as they are capable of absorbing and desorbing lithium” (paragraph 0063), and all the same, the solvent mixture is employed to solve the particular problem for the nonaqueous electrolytic cell using a carbon material for the negative electrode and it is not suggested to apply the solvent mixtures to a nonaqueous electrolytic cell using a material other than a carbon material for a negative electrode.
Further, this invention cannot suppress swelling in the nonaqueous electrolytic cell when the total content of vinylene carbonate and propylene carbonate (all cyclic carbonic acid esters) is higher than the content of the linear carbonic acid esters by vol. % and the content of vinylene carbonate is less than 3 vol. % (see Table 1 to Table 4), and therefore, only very limited solvent mixtures has to be used.
Patent Document 10: Japanese Patent No. 3410027
Although formation of a coat on the surface of the negative electrode using a carbon material is not described in Patent Documents 9 and 10, and according to the following description of Patent Document 11, even if it is apparent that a coat is to be formed on the negative electrode surface in the case of using a carbon material as a negative active material; it cannot be said that a coat is to be formed on the negative electrode surface in the case of using lithium titanate as a negative active material.
That is, Patent Document 11 describes: “it is found that the nonaqueous electrolyte secondary cell using lithium titanate as a negative active material and a carbonaceous material as a conductive agent is inferior in various high temperature characteristics such as high temperature storage characteristics, and high temperature cycle performance since reaction of the carbonaceous material and the liquid electrolyte is caused in high temperature environments and a large quantity of gas is generated. However, in the case of the nonaqueous electrolyte secondary cell using a carbon material which absorbs and desorbs lithium for the negative active material, such problems are not caused. As a result of comparison of both cells, the following is understood. In the charge-discharge cycle, when the negative active material contains a carbon material, the surface of the carbon material is covered with the coat, whereas when the negative active material contains lithium titanate, the surfaces of the lithium titanate and the carbonaceous material are not covered with the coat. Accordingly, it is considered that the coat suppresses the gas generation due to the reaction of the carbon material and the liquid electrolyte. The coat is formed at a negative electrode potential of about 0.8 V or lower to the potential of Li metal (hereinafter, the potential is a value to the potential of Li metal unless otherwise specified) and a particularly good coat is formed at a negative electrode potential of about 0.4 or higher and 0.5 V or lower. The range of the Li absorbing and desorbing potential of the carbon material absorbing and desorbing lithium is about 0.1 V or higher and about 0.9 V or lower and the negative electrode potential is lowered close to 0.1 V at the time of initial charging. Therefore, at the negative electrode potential of about 0.8 V or lower, the coat is formed by reaction of the carbon material and the liquid electrolyte and thereafter, the carbon material can exist stably. On the other hand, the Li absorbing and desorbing potential of lithium titanate is in a range of about 1.3 V or higher to about 3.0 or lower and it is considered that no coat is formed. Accordingly, in the case of a negative active material which has the Li absorbing and desorbing potential nobler than the potential of lithium metal by 1 V, represented by lithium titanate, no coat is formed on the surface and thus gas generation due to reaction of the carbonaceous material which is a conductive agent with the nonaqueous electrolyte cannot be suppressed” (paragraphs 0014 to 0017), and therefore, even if the nonaqueous liquid electrolytes described in Patent Documents 9 and 10 are employed, it is not recognized that the coat is formed on the negative electrode surface when a negative active material such as lithium titanate into/from which lithium ion is inserted/extracted at 1.2 V or higher potential to the lithium potential is used. Accordingly, it cannot be said that a person skilled in the art could have easily achieved application of the nonaqueous liquid electrolytes described in Patent Documents 9 and 10 to the nonaqueous electrolytic cell using a negative active material such as lithium titanate in order to form the coat on the negative electrode surface and suppress the gas generation.
Patent Document 11: JP-A No. 2005-317508
As described in Patent Document 11 as, “the inventors of the invention have made various investigations and accordingly have found that a good coat excellent in ion conductivity can be formed on the negative electrode surface and thus a nonaqueous electrolyte secondary cell excellent in high temperature characteristics and high current characteristics can be realized by providing a negative electrode containing lithium titanate and a carbonaceous material as well as a nonaqueous electrolyte containing a linear sulfite” (paragraph 0018), the coat is formed, by chance, on the negative electrode surface using lithium titanate as a negative active material by containing a linear sulfite in the nonaqueous electrolyte; however as shown in Examples in the present description (Comparative Example in which diethyl sulfite, a linear sulfite, is contained), the gas generation or cell swelling cannot be suppressed sufficiently only by forming the coat on the negative electrode surface.
Patent Document 12 describes a nonaqueous electrolytic cell using graphite as a negative active material (paragraphs 0037 to 0039) and as described there as “with respect to evaluation cells A1 and A2 according to the invention, since lithium-bis(oxalato)borate having reduction potential of about 1.6 to 1.7 V is used, it is considered that the good coat with high lithium ion permeability has been formed on the negative electrode surface before the phosphoric acid ester compound having reduction potential of about 1 V is reduced. Accordingly, it is considered that reduction of the phosphoric acid ester compound is suppressed and the charge/discharge efficiency is improved” (paragraph 0049); “with respect to evaluation cells X1 and X2 for comparison, no lithium-bis(oxalato)borate is contained in the liquid electrolyte and VC is added. The reduction potential of VC is about 0.9 V. Accordingly, it is considered that before a good coat is formed on the negative electrode surface by reaction of VC with the negative electrode, reduction of the phosphoric acid ester compound having reduction potential of about 1 V has started and as a result, the respective initial charge/discharge efficiencies are lowered as compared with those of the cells A1 and A2 containing lithium-bis(oxalato)borate in the liquid electrolytes” (paragraph 0050), the trial to improve the charge-discharge efficiency by forming a good coat on the negative electrode surface by reduction decomposition of an additive is known well and that the reduction potential of VC is about 0.9 V is also known well. However, with respect to the lithium titanate negative electrode, since the operating potential is 1.2 V or higher and generally the negative electrode potential never becomes 0.9 V or lower, it has not been considered to select VC having reduction potential of about 0.9 V as known well as an additive in order to form a coat on the negative electrode surface in a nonaqueous electrolytic cell comprising a lithium titanate negative electrode.
Patent Document 12: JP-A No. 2005-259592
As being understood from Examples of Patent Documents 1, 2, 11, and the like, aluminum is conventionally employed for a current collector of a lithium titanate negative electrode, but it is known well that aluminum is alloyed with lithium at a potential of 0.4 V or lower, therefore there has been no idea to carry out deep charging to 0.4 V or lower of the negative electrode potential for such a cell.